1. Field of Art
The present invention relates to a system for selecting the drive ratio of a multiple ratio mechanical gearbox in a harvester having an engine connected to drive the harvester through a transmission line that includes the said mechanical multiple ratio gearbox and having a brake for preventing movement of the harvester.
2. Description of Prior Art
The multiple ratio gearbox comprises a first shaft, a first gear mounted fast in rotation with the first shaft, a second shaft having a second gear rotatably mounted thereon, the first and second gears being in permanent meshing engagement, and a gear selector for selectively coupling the second gear for rotation with the second shaft to permit torque to be transmitted between the first and second shafts, wherein the gear selector comprises a cog fast in rotation with the second gear, a carrier fast in rotation with the second shaft, and a synchronizer surrounding the carrier and having internal teeth permanently engaging between external teeth of the carrier, the synchronizer being slidable relative to the carrier into engagement with external teeth of the cog fast in rotation with the second gear.
An important difference between a transmission used in a harvester and one used in a road vehicle is that in a road vehicle gear changes can be carried out while the vehicle is in motion, whereas in a harvester final drive gear selection is carried out with the vehicle at a standstill. Thus, while synchromesh is used in road vehicles to bring the speed of one gearbox shaft gradually up to the speed of another shaft to effect smooth gear changes, in a harvester a synchronizer is used not for speed matching but for position or phase matching to ensure that the teeth of the carrier are correctly aligned with the teeth of the cog that is driven by the first shaft.
Because of the construction of the gearbox of a harvester, it is essential to ensure that the harvester is at a complete standstill before making any attempt at gear changing. With a combine harvester in which the transmission line comprises a hydraulic transmission in addition to the mechanical gearbox, the driver conventionally moves the hydrostatic handle into a neutral position to bring the vehicle to a stop before selecting a change of gear. In most cases however, and most certainly when on a slope, it was also expected of the driver to also operate a foot brake to ensure that the vehicle was brought and maintained at a perfect standstill before selecting a change of gear. However, experience taught that because of the extra effort required of the driver to operate the foot pedal, this would sometimes not be carried out correctly impeding a proper change of gear and obliging the driver to make another attempt before the required gear selection could be effected.
The present invention seeks to provide a method and system for gear selection in a harvester that place fewer demands on the driver and ensure a smooth gear change.
In accordance with a first aspect of the invention, there is provided a method of selecting a final drive gear ratio in a harvester that comprises selecting the desired ratio by manually depressing a button on a control panel, automatically activating a brake to bring and/or maintain the harvester at a standstill in response to the depression of the button on the control panel, and activating a gear selection mechanism to select the desired gear ratio only after the harvester has come to a complete stop.